254 BRITISH GRAPTOLITES. 



u]) ward; its aperture is concave, and appears to be prolonged into a spine. 

 Th. I 2 develops from th. I 1 , growing at first horizontally across the sicula, and 



Fias .ma-c.-Giyptogra P tus dentatus then upward and outward ; in this case the spine 

 (Brongniart). seems to be given off near where the change in 



direction of growth takes place, but it is hard to 

 determine where these spines actually originate. 

 n The septum makes its first appearance after two 



theca? have developed on each side of the sicula; 

 the septal groove is somewhat flexed at first, but 

 becomes straighter distally. 

 a All the thecas subsequently developed show 



flowing sigmoid curvature of their ventral walls in 

 {, bi-profile view, and the entire apertural region is 



a. Proximal end, obverse aspect showing somewhat introtorted, so that the apertural margins 



sicula. Cefn-farchen farm, near _ — 



Lianfanteg station. Upper Arenig. appear approximately horizontal, but with a decided 



Geol. Surv. England. rr 1 l . J , 



b. Proximal end, reverse aspect showing tendency to introversion. Judging from examples 



virgella. Enlargement of part of . ...... , , , , , . 



pi. xxxi, fig. 4c. preserved in low relief it is probable that this is 



r. Distal theca? in low relief. Llan, near n . ...... Pjl . -. . , , 



Lianfanteg Station. Upper Arenig. due to a Certain twisting ot the axis ot the theca, 



this form pointing in the direction of the sub-group 

 of Glyptograpti next to be described. 



Affinities. — G. dentil f us unites in itself many characters found distinct in later 

 forms. The polypary seems to have had a certain degree of concavo-convexity, 

 bringing it near to that of the Mesograpti. Although in the general form of the 

 thecas it agrees essentially with other Glyptograpti, yet the lobate character of the 

 apertural margin is more pronounced than usual, and approaches more nearly to 

 that of the Orthograpti. On the whole it apparently presents most analogies with 

 Glyptog. teretiusculus, and we therefore group it with the Glyptograpti, but as an 

 extreme form. It may be distinguished from Glyptog. teretiusculus by having 

 smaller and more closely set thecas in the proximal region of the polypary, and by 

 the introtorsion of the whole apertural region of its thecas. 



Horizons and Localities. — Upper Arenig and Llanvirn; Upper Skiddaw Slates 

 (Ellergiil Beds). 



Shropshire : Shelve Church. N. Wales: Pont Seiont; Peblig Mill ; Arenig, etc. 

 S. Wales : Porth Hayog; Llanvirn Quarry; Melin-y-castell, St. Clears; Llandeilo, 

 Abercowin ; Rhyd-y-behan Cottage, near Clynderwen railway station ; near Llan- 

 fallteg railway station, etc. Lake District: Thornship Beck; Aik Beck, Pooley; 

 Troutbeck; Ellergiil, Milburn ; N. W. Longside, Skiddaw; Mosedale Beck; 

 Alungrisedale ; Glenderamakin River; Bassenthwaite Sand-beds; Outerside ; 

 Master Sike, Crossfell. 



Associates, etc. — Glyptog. dentatus is the oldest Diplograptus hitherto met with 

 in Britain ; it makes its appearance in the Arenig Rocks (zone of Vidymog. 



